An ultrasonic ranging device may be used to detect the presence of and/or distance to a target object, such as an object located underground or stored in an enclosed container. The ultrasonic ranging device typically operates by generating an ultrasonic signal or pulse, which may be reflected upon striking the target object. The ranging device may listen for the reflected ultrasonic signal, or echo signal, and measure the period between the time the signal is sent and the time the echo signal is received. By measuring this time period, the ranging device may determine the distance between the device and the target object. For example, the ranging device may be placed at the top of a storage tank in order to measure the distance from the top of the tank to the top surface of a material in the tank. Thus, the ranging device may be used to determine the level of material in the tank.
Some ultrasonic ranging devices may include a transducer for both generating the ultrasonic signal and receiving, or detecting, an echo signal that bounces back from the target object. Such devices often have a parameter called “near zone,” which may refer to the minimum distance that can be measured by the ranging device. In other words, the ranging device may not be capable of measuring the distance to a target object that is located within the near zone.
The near zone limitation may be caused by a “ringing” effect that frequently occurs in the transducer after it generates an ultrasonic signal or pulse. Ringing refers to a series of (decaying) signals that are usually generated by the transducer after an input electrical signal is no longer applied to the transducer. A larger input electrical signal may increase both the amplitude of the generated ultrasonic signal and the ringing time in the transducer (e.g., the time it takes the transducer to get up to 90% of maximum amplitude, or down to 10% above zero amplitude). Ringing in the transducer may limit the transducer's ability to detect an echo signal from a close target object due to signal interference (i.e., the amplitude of the echo signal may be smaller than that of the decaying signals). Thus, the ranging device may be incapable of both detecting the presence of and determining the distance to the target object, thereby limiting the utility of the ranging device. Therefore, a need exists for an ultrasonic ranging device that overcomes these difficulties to permit ultrasonic ranging in the near zone.